The invention generally relates to hip implants, particularly to a modular hip implant comprising a base member in form of an acetabular support shell for being attached to a pelvic bone, and comprising a cup for receiving a hip joint prosthesis, wherein the base member can be attached at the pelvic bone and comprises an accommodation portion for the cup, wherein the cup has an outer shape complementary to the accommodation portion.
Artificial hip joints comprise a hip joint prosthesis insertable into the femur and an acetabular cup to be attached to the pelvic bone. The end of the hip joint prosthesis pointing away from the knee has a spherical shape to engage into the acetabular cup of a hip implant and to form a ball joint together with this cup.
If the pelvis does not offer sufficient osseous substance for safely fastening a simple acetabular cup, an additional support by ring or shell structures is required. These structures can particularly be used in hip joint exchange operations, in which the actual bone loss can only be determined during an operation and after the primary implants have been disassembled. The bone cement used to fix the cup in place also represents one reason for exchanging a cup. Also in the case of a successful cup revision, a later second revision should be expected. Other than the reconstruction of osseous defects by using transplants, an easily-handled, purely cement-free fastening therefore offers significant advantages.
Important aims in the placement of an artificial acetabular cup or revision cup are:
1. primary stable fastening of bone transplants,
2. primary stable fastening of the acetabular cup or support cup,
3. reconstruction of the anatomic hip rotation center, and
4. correct tilting of the cup in the so-called angles inclination and anteversion.
If the two centers of rotation of the two hip joints are connected by a first (horizontal) axis, the anteversion represents the angle of the cup axis with respect to the first (horizontal) axis in a (horizontal) plane through the first (horizontal) axis and a second anterioposterior (horizontal) axis. The inclination represents the angle of the cup axis with respect to the first (horizontal) axis in a (vertical) plane through the first (horizontal) axis and a craniocaudal (vertical) axis. In the human body, the inclination has values in the range between 30° and 50°, whereas values in the range of 5 to 25° are measured in anteversion.
The position of the hip rotation center determines the biomechanics of the hip joint and thus also the life of the implanted endoprosthesis. Additionally, the scope of movement and the transmission of power are defined by the anteversion and by the inclination. If the anteversion is incorrectly chosen, the neck of the femoral shaft prosthesis can abut at the cup edge or the support shell and lever the joint ball out of the cup/shell structure. In the case of an incorrect inclination with a lack of roofing of the joint ball by the acetabular cup, the risk of dislocating the hip joint exists.
When implanting an artificial hip joint, the surgeon should be prepared to take the individual anatomic conditions of the body to be treated into consideration, including the skeletal structure with osseous defects and the muscle/ligament situation of the supporting ligaments. The above-mentioned interrelationships, however, frequently force a compromise between primary stable fastening of support shells at the bone, optimum position of the hip rotation center and correct inclination and anteversion angles. Such difficult implant exchange situations make simple handling with a fully cement-free supply along with an extended adjustment possibility of the anteversion and inclination especially desirable.
Conventional hip implants for revisions are composed of a shell-shaped base member, which predominantly takes over a support function. These implants are usually fastened without cement in a primary stable manner on the pelvic bone. A cup-shaped insert can be inserted in cement-free or cemented manner into this cup by taking the correct inclination and anteversion angle into consideration. This insert accommodates the ball of the hip joint prosthesis.
The cemented fastening of the cup has the advantage that this cup can be adjusted autonomously within certain limits in the shell. The additional introduction of bone cement is, however, undesirable in many cases because of the known disadvantages, and it does not offer a durable support in the shell structure against oscillations occurring between the shell and the cup.
An acetabular socket support ring is known from U.S. Pat. No. 5,425,778, which permits adjusting the anteversion and inclination of a cement-free cup. Therein, the joint ball is retained and held in position between an inner concavely shaped supporting ring and a second ring that can be screwed into the supporting ring, the ring also being shaped concavely on the section facing the joint ball. A disadvantage of this solution is that when the cup is tilted, it is submersed under the edge level of the supporting ring (U.S. Pat. No. 5,425,778: FIG. 2) and thus the neck of the shaft of the femur can abut against the edge of the supporting shell. This creates the risk that the joint ball can be levered out of the cup. Additionally, this risk cannot be avoided by providing recesses in the supporting ring, because they are connected with a loss of the internal thread in the same. Furthermore, it may occur that individual parts of the supporting ring after operation are twisted relative to each other as a result of heavy mechanic load so that an unfavorable anteversion and/or inclination results. Furthermore, it may be difficult to correctly adjust anteversion and inclination during an operation.
It is an object of the present invention to provide a modular hip implant, which simplifies operation technique when inserting a hip implant into the body to be treated and which can better be adapted to the anatomic requirements of a patient.
A further object of the invention is to provide a shell structure that can be attached at the bone without using cement, in which the bearing insert of a hip joint prosthesis can be introduced (either cemented or cement-free), and which allows the safe adjustment of inclination and anteversion angles also when being attached without cement with a simplified operation technique.
Yet another object of the invention is to provide a modular hip implant with an improved stability.